In order to satisfy the high precision demands made in respect of modern machine systems, in particular program controlled machine tools, it is necessary for the employed measuring systems to operate as error-free as possible and independently of varying operating conditions. In order to prevent contamination of the measuring systems as far as possible, these systems are normally disposed quite remote from the operating range of the machine. But due to thermal expansion effects of the individual machine parts caused by temperature variations, which occur for example during start-up and until an approximately constant operating temperature has been reached, displacements are thereby caused between the work spindle or the machining tool and the measuring system.
German Pat. No. 2,450,322 discloses an apparatus generally of the specified kind for compensating for thermal expansion of a part of a machine tool, in which a strain rod carrying the actual rule is mounted in close engagement on a side wall of a slide, for example, a headstock, which is exposed to the temperature variations. The material of the strain rod is selected such that in the respectively occurring temperature ranges the thermal expansion of the strain rod is greater by a certain amount than that of the slide. The rear end of the strain rod is rigidly mounted on the rear end of the slide or headstock, which rear end is heated only slightly. Upon heating of the slide or headstock, such as during start-up, the increased elongation of the strain rod will compensate for the elongation of the machine parts projecting beyond the slide, for example, of a vertical milling head attached to the headstock, which elongation cannot be directly detected by the measuring system. The rule carrying the indices for a scanning head secured to the stationary machine part is secured with its forward end to the forward end of the strain rod by use of a thermally insulating intermediate layer which insulates the rule relative to the heated parts and is intended to prevent thermal expansion of the rule.
But with this prior compensating system problems may arise due to the fact that the strain rod is fixedly joined to the machine part only at its rear end while its forward end is merely retained in a guide means mounted on the headstock. Since the strain rod therefore contacts the machine part so as to be relatively movable in longitudinal direction, a continuous positive contact along the entire length of the strain rod cannot be achieved, thus resulting in a not exactly defined heat transmission between the machine part (headstock) and the strain rod. Especially due to changes of length of the strain rod, which is made from a mechanically relatively flexible material such as aluminum, there is the risk upon assembly or during operation that the continuous surface contact with the machine part is disturbed, thus resulting in significant changes of the heat transmission and therefore of the strain behavior of the rod. Moreover, the length dimension of the respective strain rods must also be matched with the different machine parts or types of machines.
For compensating for the thermal expansion of a headstock it is known from German Pat. No. 1,010,802 to provide a strain rod in parallel to the headstock and to securely fix it on one side to the machine stand and on the other side to a machine part carrying the headstock. With that structure, upon a change of temperature the entire headstock or the machine part carrying the same will be displaced due to the corresponding thermal expansion of the strain rod. This compensating system is extremely expensive, because the strain rod is integrated in the machine itself. Retrofitting of machine tools with this compensating system is not reasonably possible.